The present invention relates to the locating of an open circuit in a cable conductor.
A number of systems have been developed for monitoring the integrity and locating faults on communications cables, including fibre optic cables. One weakness of the systems developed to date has been the inability to identify the exact location of an open circuit in a monitored cable conductor. The open circuit is important to identify because if left unattended, a large section of the cable may become unmonitored, thereby reducing the effectiveness of the monitoring system.
U.S. patent application Ser. No. 09/092,849, filed Jun. 8, 1998 describes a system for locating a cable break in a conductive shield or armour of a cable. The shield is divided into sections along the cable and each section is monitored by an instrument located at the end of the section. The present application is concerned with a method for identifying the location of an open circuit in an unsegmented cable conductor, thereby helping enhance the performance and effectiveness of a monitoring system. Used in conjunction with existing cable monitoring apparatus or as a stand-alone system, the cable open locating apparatus will greatly reduce time to locate a cable xe2x80x9copenxe2x80x9d.
Many cables are located near sources AC voltage, usually 60 Hz or 50 Hz from power mains and may have those frequencies induced in the monitored cable conductor. The preferred location method and apparatus should allow the rejection of a specified frequency and its harmonics. This can be particularly important for locating a fault in a fibre optic cable which follows the same right-of-way as high power electric utility cables.
According to one aspect of the present invention there is provided a method of locating an open circuit fault in an electric conductor extending along a cable and having a unit capacitance per unit length, said method comprising:
applying a step function DC voltage to one end of the conductor;
measuring the DC current along the conductor over a sampling period;
integrating the DC current measured over the sampling period; and
computing the distance between said one end of the conductor to the open circuit fault from the integrated DC current and the unit capacitance of the conductor.
For communications cables, various transmission parameters of the cable are known or can be determined. For example, with telephone long distance fibre optic cable, the parameters resistance per unit length and capacitance per unit length of the metallic cable shield are known. By capacitively charging the cable with a step function voltage, measuring and integrating the current, the charge on the shield and the total line capacitance can be calculated. Using the known capacitance per unit length, it is then possible to calculate the length of the cable to an open circuit fault.
The sampling period is sufficiently long for the DC current to reach a substantially steady state, representing a full capacitive charge on the conductor.
The DC current in the conductor is preferably measured by sampling the current at a predetermined sampling frequency. The sampling frequency is preferably an integral multiple of the local AC mains power frequency, either 60 Hz or 50 Hz, to eliminate the effect of induced AC voltages in the calculations.
To eliminate the effects of a resistive fault, the method may include the steps of determining the conductor resistance and subtracting from the calculation of capacitive charge a DC fault current that is calculated from the conductor resistance and the monitored DC voltage.
The conductor resistance may be determined by applying a DC voltage to the conductor and calculating the resistance from steady state values of the DC voltage and DC current. It is preferred to carry out multiple measurements of resistance using reversed polarity DC voltages. This cancels any diode effect in a resistive fault.
According to another aspect of the present invention there is provided, an apparatus for locating an open circuit fault in an electric conductor extending along a cable and having a unit capacitance per unit length, said apparatus comprising:
a power supply for applying a step function DC voltage to one end of the conductor;
a current meter for measuring the DC current along the conductor over a sampling period;
a processor for integrating the DC current measured over the sampling period and computing the distance between said one end of the conductor to the open circuit fault from the integrated DC current and the unit capacitance of the conductor.
The processor may include means for sampling the current at a predetermined sampling frequency.
The apparatus preferably includes a voltmeter for measuring the DC voltage at the one end of the conductor. The processor may include means for determining the conductor resistance using the voltmeter output and means for integrating the difference between the measured DC current over the sampling period and the resistive DC current calculated from the conductor resistance and the monitored DC voltage.